I do 12" blocks all the time and I have yet to hear of a 12-1/2" finished block. Mine are always 12-1/2" unfinished and 12" finished. That being the case, that may be why your blocks aren't right! Check that one out. It may be it should be 12-1/2" unfinished, 1/4" off for seams, you have a 12" block.
Edie